Field of the Invention
The present invention relates to an organic light emitting display device, and more particularly, to an organic light emitting display device and a method of driving the same, which optimize a driving voltage of a data driver to reduce power consumption.
Discussion of the Related Art
General organic light emitting display devices include a display panel, which includes a plurality of pixels respectively formed in a plurality of pixel areas defined by intersections between a plurality of data lines and a plurality of gate lines, and a panel driver that emits light from the plurality of pixels.
A compensation scheme is categorized into an internal compensation scheme and an external compensation scheme depending on a position of a circuit that compensates for a characteristic deviation of pixels. The internal compensation scheme is a scheme in which a compensation circuit for compensating for a characteristic deviation of pixels is disposed inside each of the pixels. The external compensation scheme is a scheme in which the compensation circuit for compensating for a characteristic deviation of pixels is disposed outside each pixel.
FIG. 1 is a circuit diagram for describing a pixel structure based on an internal compensation scheme of a related art organic light emitting display device.
Referring to FIG. 1, each of a plurality of pixels formed in a display panel includes a switching thin film transistor (TFT) ST1, a driving TFT DT, a capacitor Cst, an organic light emitting diode OLED, and a compensation circuit that compensates for a change in a characteristic (a threshold voltage and mobility) of the driving TFT.
The first switching TFT ST1 is turned on according to a gate driving signal (a scan signal) supplied to a corresponding gate line GL. The first switching TFT ST1 is turned on, and thus, a data voltage Vdata supplied to a corresponding data line DL is supplied to the driving TFT DT.
The driving TFT DT is turned on with the data voltage Vdata supplied to the first switching TFT ST1. A data current Ioled flowing to the organic light emitting diode OLED is controlled with a switching time of the driving TFT DT. A driving voltage EVDD is supplied to a power line PL, and when the driving TFT DT is turned on, the data current Ioled is applied to the organic light emitting diode OLED.
The capacitor Cst is connected between a gate and source of the driving TFT DT. The capacitor Cst stores a voltage corresponding to the data voltage Vdata supplied to the gate of the driving TFT DT.
The organic light emitting diode OLED is electrically connected between the source of the driving TFT DT and a cathode voltage EVSS. The organic light emitting diode OLED emits light with the data current Ioled supplied from the driving TFT DT.
However, the threshold voltage (Vth) and mobility characteristics of the driving TFTs DT of the respective pixels are differently shown due to a non-uniformity of a TFT manufacturing process. For this reason, in general organic light emitting display devices, despite that the same data voltage Vdata is applied to the driving TFTs DT of the respective pixels, since a deviation of currents flowing in the respective organic light emitting diodes OLED occurs, it is unable to realize a uniform image quality.
To solve such problems, a compensation circuit is provided in each pixel. The compensation circuit senses the changes in a threshold voltage “Vth” and mobility “k” of the driving TFT of each pixel, and compensates for the changes in the threshold voltage “Vth” and mobility “k”. Therefore, a driving voltage “Vdata+Vth” that is obtained by summating a compensation voltage “Vth” and a data voltage Vdata based on an image signal is supplied to a gate of the driving TFT.
The related art organic light emitting display device controls a level of the data current Ioled, which flows from a first driving voltage EVDD terminal to the organic light emitting diode OLED, by using a switching time of the driving TFT DT. Therefore, the organic light emitting diode OLED of each pixel emits light, thereby displaying an image.
FIGS. 2 and 3 are diagrams illustrating a related art SVDD voltage setting method based on the external compensation scheme.
Referring to FIGS. 2 and 3, a driving voltage supplied to a driving TFT is obtained by summating a compensation voltage and a data voltage Vdata based on an image signal. The compensation voltage “initial compensation voltage+sequential compensation voltage” is obtained by summating an initial compensation voltage, used to compensate for an initial deviation, and a sequential compensation voltage which is used to compensate for a sequential change such as deterioration or a characteristic change during a use period. An SVDD value that is a driving voltage of a data driver is determined according to the maximum value of a driving voltage supplied to the driving TFT. An initial compensation region and a sequential compensation region is not clearly divided in a compensation voltage, and a voltage range obtained by subtracting an initial compensation voltage range from a total compensation voltage range is used as the sequential compensation voltage.
In a related art organic light emitting display device based on the internal compensation scheme, the sum of the compensation voltage “Vth” (generated by the compensation circuit of a pixel) and a data voltage Vdata input to the pixel is applied to the driving TFT. In the internal compensation scheme in which the compensation circuit is provided in each pixel, the compensation voltage is added in each pixel, and thus, the same driving voltage is applied irrespective of the threshold voltage and the mobility.
As illustrated in FIG. 2, the SVDD voltage that is the driving voltage of the data driver is set to a fixed value irrespective of the compensation voltage “Vth”. Since the SVDD voltage is fixed and used, the voltage remaining for sequential compensation in the compensation voltage is not actually used, and the SVDD voltage is set as a high voltage, thereby wasting power. For example, when it is assumed that the data voltage Vdata is 10 V, the compensation voltage is 8 V, the initial compensation voltage is 2 V, and the SVDD voltage is 18 V, only a voltage of 12 V is initially used in the SVDD voltage of 18 V. That is, a voltage of 6 V is consumed without being used.
Moreover, as illustrated in FIG. 3, the SVDD voltage is changed according to an average picture level (APL) of the data voltage Vdata. In this case, the SVDD value is changed by reacting on a change in the data voltage Vdata with respect to the maximum compensation voltage, irrespective of the threshold voltage “Vth” and the mobility “k”. Therefore, as the APL becomes higher, a ratio of an unused compensation voltage in a total SVDD voltage increases, and thus, consumption power that is wasted without being actually used increases.